1. Field of the Invention
The present invention relates to a surface acoustic wave resonator, and a surface acoustic wave filter and an antenna duplexer in which the surface acoustic wave resonator is used.
2. Background of the Invention
In order to realize a surface acoustic wave filter advantageously characterized in its broadband, a piezoelectric substrate having a large electromechanical coupling coefficient, such as a lithium niobate substrate (LiNbO3), was conventionally used. However, the surface acoustic wave filter, in which the substrate of this type is used, was generally disadvantageous because of its poor temperature characteristic. In order to improve the temperature characteristic, there is a proposed constitution wherein a SiO2 thin film layer is formed on the LiNbO3 substrate, and a value of H/λ is 0.115-0.31 provided that a cut angle of a rotor Y-cut as the LiNbO3 substrate is −10 to +30 degrees, a thickness dimension of the thin-film layer is H, and a wavelength of an operation center frequency of the surface acoustic wave is λ (for example, see the Patent Document 1).
When the surface acoustic wave resonator, which is formed on the substrate is connected to the ladder type, the surface acoustic wave filter having the broadband characteristic can be realized. In the case where the substrate of the foregoing type is used, there may be the spurious in the transverse mode. A conventional method of controlling the spurious is to weight an interdigital transducer electrode. FIG. 15A is a top view of a constitution of a conventional surface acoustic wave resonator. FIG. 15B is a sectional view of 15B-15B part shown in FIG. 15A. Interdigital transducer electrode 1202 and reflector electrodes 1203 are formed on piezoelectric substrate 1201, and SiO2 thin film 1204 is further formed thereon. Interdigital transducer electrode 1202 is apodization-weighted in order to control the spurious in the transverse mode.
However, the conventional surface acoustic wave resonator has a problem that the characteristics thereof are deteriorated by the leak of the surface acoustic wave in the transverse direction resulting from an acoustic velocity in the surface acoustic wave resonator. Further, in the case where the surface acoustic wave filter comprises the surface acoustic wave resonator, an insertion loss and a deteriorated attenuation characteristic are unfavorably generated in the surface acoustic wave filter.    [Patent Document 1] Unexamined Japanese Patent Publication No. 2003-209458.